1. Field of the Invention
The present invention relates to an optical pickup, and more particularly, the present invention relates to an optical pickup for spotting lights emitted from two light sources having different wavelengths onto an optical disc, which adjusts an astigmatic difference of one light source emitting a light through a flat beam splitter, thereby to reduce aberrations of beams spotted onto the optical disc.
2. Description of the Related Art
Generally, optical discs are widely used these days because they can record and reproduce information at a relatively high density when compared to a long play record (LP) or a magnetic recording tape and can be kept semi-permanently. The technology of compact discs (CD) is most widely spread and such discs are used as optical discs. However, since compact discs have a limited recording capacity, they cannot be reliably used for movies, music, games, or other multimedia which have a playing time exceeding, for example, 90 minutes. For this reason, it is necessary to prepare two or more discs and have them run successively.
To resolve this problem, a digital video disc (DVD) is recently developed as a next generation optical disc. A DVD has a memory capacity which is 25 times that of a CD and can read ten million bit data per second. This is because the DVD has pits and tracks which correspond to halves of those of a CD. In other words, because a pit and a track of the DVD are about 0.4 feet and 0.8 micron, respectively, a high density recording is possible. Accordingly, due to this high capacity and operational capability, the DVD can be easily used in various fields such as movies, music, games or other multimedia.
On the other hand, while an optical pickup can be provided for exclusively playing back the DVD, it is preferred that both a CD and a DVD be played back by the same player in view of economics. Accordingly, a demand for an optical pickup which can play back a CD and a DVD, is gradually increasing among consumers.
Referring to FIG. 1, there is shown a schematic view of an optical pickup of the related art, which uses two light sources in order to play back both a CD and a DVD. The optical pickup of the related art includes a first light source 1 for recording and reproducing information onto and from a digital video disc D using a laser diode which emits a beam of 650 nm wavelength. The optical pickup also includes a second light source 2 for recording and reproducing information onto and from a compact disc D' using a laser diode which emits a beam of 780 nm wavelength. The second light source 2 is positioned such that it is parallel to the first light source 1.
A flat beam splitter 3 is arranged on a path of the beam emitted from the first light source 1, and a cubic beam splitter 4 is arranged on a path of the beam emitted from the second light source 2. The optical pickup includes a reflecting lens 9 for directing beams which are emitted from the first and second light sources 1 and 2 and then reflected from the flat beam splitter 3 and the cubic beam splitter 4, toward the digital video disc D and the compact disc D', and a collimator lens 5 for shaping the beams reflected from the reflecting lens 9. The optical pickup further includes an objective lens 6 for spotting the beams shaped by the collimator lens 5 onto the digital video disc D and the compact disc D', and a photodiode 8 for detecting recorded information and an error from the beams reflected from the digital video disc D and the compact disc D'.
The optical pickup of the prior art, constructed as mentioned above, has an advantage in that stable recording and reproducing operations are ensured. This is because an aberration is generated only in a small amount during the travel of the beams emitted from the first and second light sources 1 and 2 and reflected by the flat beam splitter 3 and the cubic beam splitter 4 to be spotted onto the optical discs.
However, since the costly cubic beam splitter 4 must be used in order to obtain such a stable spot, expenses of the optical pickup are increased. Also, since the first and second light sources 1 and 2 are disposed in side-by-side relationships with the flat beam splitter 3 and the cubic beam splitter 4, respectively, a relatively wide installation space is needed and it is difficult to manufacture a compact optical system.
Referring to FIG. 2, there is shown a schematic view of another optical pickup of the related art, which uses two light sources. The optical pickup uses an anisometrical hologram beam splitter 3a. In this respect, first and second light sources 1 and 2 are slopingly mounted such that they define predetermined angles to a plane of the hologram beam splitter 3a.
On a path of the beams which are emitted from the first and second light sources 1 and 2 and passes through the hologram beam splitter 3a, there are sequentially disposed a flat beam splitter 4a, a collimator lens 5 for shaping the beams, and an objective lens 6 for spotting the beams onto optical discs. The flat beam splitter 4a is slopingly mounted. The optical pickup also includes a photodiode 8 for detecting recorded information and an error signal from the beams reflected by optical discs. The photodiode 8 is disposed such that it is opposite to the flat beam splitter 4a.
While the optical pickup of the related art, constructed as mentioned above, has an advantage in that without using a cubic beam splitter, expenses are reduced, it still suffers from defects in that, due to wavelength variations of the beams emitted from the first and second light sources 1 and 2, a flickering of an optical axis is caused in an entire optical system. That is, in the case of a conventional laser diode, a wavelength variation through .+-.20 nm is caused, due to a variation in a surrounding temperature of the optical system.
Accordingly, in the case where the anisometrical hologram is used as described above, since angles of a wave front and a wave tail are changed so that a tilt of a light axis exceeding 0.2.degree. is caused at an exit side of the light, a configuration of a light spotted onto the optical disc is deteriorated. At the same time, due to the fact that a position of a light spotted onto the photodiode is changed relying upon a wavelength, in the case where the light is focused onto the photodiode, it is impossible for the optical system to calculate a focusing error of the optical disc in terms of quantity of light which is spotted onto a portion of the 4-divided photodiode. Namely, since a quantity distribution of the light which is spotted onto the photodiode is lop-sided, it is impossible to calculate through the photodiode, whereby a focusing error is caused.
Referring to FIG. 3, there is shown a schematic view of still another optical pickup of the related art, which uses two light sources. This type of optical pickup is disclosed in Japanese Patent Laid-Open Publication No. Heisei 6-150363. In the structure of the optical pickup, first and second light sources 101 and 102 are arranged such that they are orthogonal to each other. On a point where beams emitted from the first and second light sources 101 and 102 cross with each other, there is positioned a phase plate 103 for passing and reflecting the beams. Also, collimator lenses 104 and 105 for shaping the incident beams into parallel lights are positioned in front of the first and second light sources 101 and 102, respectively.
A beam splitter 106 for reflecting the beams onto an optical recording medium D is positioned in front of the phase plate 103. An objective lens 107 is provided as means for spotting the beams reflected by the beam splitter 106 onto the optical recording medium D. The objective lens 107 is located above the beam splitter 106. Polarization light splitting means 108 for dividing the beams reflected from the optical recording medium D into a main beam and a sub beam is located below the beam splitter 106. A photodiode 109 onto which the main beam and sub beam divided by the polarization light splitting means 108 are spotted, is located below the polarization light splitting means 108 as photodetecting means for separately detecting light intensities of the main beam and the sub beam.
In the optical pickup constructed as mentioned above, since the beams emitted from the first and second light sources 101 and 102 and having different light intensities are separately detected by the photodetecting means, that is, the photodiode 109, generation of cross torque is suppressed, whereby it is possible to realize an optical head of high density. However, although the beams emitted from the first and second light sources 101 and 102 have different light intensities, because they have the same wavelength, the optical pickup can record and reproduce information to and from only one optical recording medium. Hence, it is impossible for the optical pickup to be adapted for recording and reproducing information to and from both a CD and a DVD as demanded in the present invention.